Search NASA⌕ Search

Engineering topics

Heuer, A. H.

Publications and source records attributed to Heuer, A. H..

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

On the orthorhombic phase in ZrO2-based alloys

During TEM observation, a tetragonal (t) to orthorhombic (o) phase transformation often occurs in thin portions of ZrO2-containing foils. This transformation is stress-induced and in some senses artifactual, in that the reaction product is actually a high-pressure phase, relative to monoclinic (m) ZrO2, that can form from metastable t-ZrO2 in the TEM because its density is intermediate between t- and m-ZrO2. Examples of the formation of o-ZrO2 in a number of different systems are given.

Heuer, A. H.↗
NASA NTRS1985

Microstructural aspects of zirconia thermal barrier coatings

Various combination of plasma-sprayed bond coatings and zirconia ceramic coatings on a nickel-based superalloy substrate were tested by static thermal exposure at 1200 C and cyclic thermal exposure to 1000 C. The bond coats were based on Ni-Cr-Al alloys with additions of rare earth elements and Si. The ceramic coats were various ZrO2-Y2O3 compositions, of which the optimum was found to be ZrO2-8.9 wt percent Y2O3. Microstructural analysis showed that resistance to cracking during thermal exposure is strongly related to deleterious phase changes. Zones depleted of Al formed at the bond coat/ceramic coat interface due to oxidation and at the bond coat/substrate interface due to interdiffusion, leading eventually to breakdown of the bond coat. The 8.9 percent Y2O3 coating performed best because the as-sprayed metastable tetragonal phase converted slowly into the low-Y2O3 tetragonal plus high-Y2O3 cubic-phase mixture, so that the deleterious monoclinic phase was inhibited from forming. Failure appeared to start with the formation of circumferential cracks in the zirconia, probably due to compressive stresses during cooling, followed by the formation of radial cracks due to tensile stresses during heating. Cracks appeared to initiate at the Al2O3 scale/bond coat interface and propagate through the zirconia coating. Comparisons were made with the behavior of bulk ZrO2-Y2O3 and the relationship between the microstructure of the tetragonal phase and the phase diagram. A separate investigation was also made of the ZrO2-Al2O3 interface.

Mitchell, T. E.↗
NASA NTRS1984

Bubble formation in oxide scales on SiC

The oxidation of alpha-SiC single crystals and sintered alphaand beta-SiC polycrystals has been investigated at elevated temperatures. Bubble formation is commonly observed in oxide scales on polycrystalline SiC, but is rarely found on single-crystal scales; bubbles result from the preferential oxidation of C inclusions, which are abundant in SiC polycrystals. The absence of bubbles on single crystals, in fact, implies that diffusion of the gaseous species formed on oxidation, CO (or possibly SiO), controls the rate of oxidation of SiC.

Mieskowski, D. M.↗
NASA NTRS1978

Beta-to-alpha transformation in polycrystalline SiC. II - Interfacial energetics

A phenomenological analysis of the energetics of the beta-to-alpha transformation in polycrystalline SiC is presented. It is found that the extreme anisotropy of the interfacial energy between alpha- and beta-SiC can account for the rapid growth of composite grains into the beta matrix during conventional sintering or hot-pressing processes. The composite grains consist of alpha-SiC plates 'sandwiched' between well-oriented and recrystallized beta-SiC 'envelopes'. The interfaces involving the 111 plane type of beta and (0001) of alpha have much lower energies than random beta/alpha interfaces.

Mitchell, T. E.↗
NASA NTRS1977

Shock-produced olivine glass - First observation

Transmission electron microscope (TEM) observations of an experimentally shock-deformed single crystal of natural peridot, /Mg(0.88)Fe(0.12)/2SiO4, recovered from peak pressures of about 56 billion pascals revealed the presence of amorphous zones located within crystalline regions with a high density of tangled dislocations. This is the first reported observation of olivine glass. The shocked sample exhibits a wide variation in the degree of shock deformation on a small scale, and the glass appears to be intimately associated with the highest density of dislocations. This study suggests that olivine glass may be formed as a result of shock at pressures above about 50 to 55 billion pascals and that further TEM observations of naturally shocked olivines may demonstrate the presence of glass.

Jeanloz, R.↗
NASA NTRS1973

Electron petrography of Apollo 14 and 15 breccias and shock-produced analogs

Six lunar breccias were studied in detail by optical and transmission electron microscopy and separated into two classes on the basis of presence or absence of recovery and recrystallization textures: Class A breccias with no recrystallization include samples 14063, 14301, and 15498; Class B breccias with recrystallization include samples 14321, 15455, and 15418. Samples of Apollo 16 soil (65010) experimentally shocked to 50, 100, and 250 kilobars pressure were sectioned and studied for comparison with the breccias. It is concluded that the Class A breccias were formed from unconsolidated lunar regolith by deformation due to shock waves of a few tens of kilobars. The Class B breccias could also have been produced from unconsolidated material at higher shock stresses, the annealing being achieved by the heating resulting from the shock deformation.

Christie, J. M.↗
NASA NTRS1972

Phase transformations and exsolution in lunar and terrestrial calcic plagioclases.

A coarse domain structure had been discovered in plagioclase in an electron microscopic study of lunar rocks reported by Christie et al. (1971). The domain structures are described in more detail, and their significance in the light of current knowledge concerning the crystal structures of the plagioclases is discussed. An exsolution texture is also described. On the basis of the scale of exsolution it appears that the Apollo 11 and 12 basalts examined cooled more rapidly than the Apollo 14 basalt, which in turn cooled more rapidly than the Stillwater rock.

Heuer, A. H.↗
NASA NTRS1972

Deformation of lunar and terrestrial minerals.

The deformation substructures observed by TEM in breccia returned by the Apollo 14 mission are illustrated and their significance is discussed in the context of earlier work on mineral deformation. Shock-produced glass and heavily deformed mineral grains indicate severe shock deformation. Extensive local recovery and sub-microscopic recrystallization are evident and suggest that the coherency of the breccias is caused by a 'shock sintering' mechanism.

Christie, J. M.↗
NASA NTRS1972

Electron petrography of Apollo 14 and 15 rocks.

Comparative optical and high-voltage electron petrographic analyses of igneous rocks and several types of breccias the Fra Mauro and Apennine sites of Apollo 14 and 15 are reported and interpreted. The crystalline rocks are found to differ substantially from the Apollo 11 and 12 basalts. Substructures in the Apollo 14 breccias are complex. At least two bonding mechanisms can be discerned from the electron petrographic examination: (a) 'shock sintering' due to high pressures and temperatures associated with the shock waves of meteoritic impacts; and (b) lithification by glass bonding.

Lally, J. S.↗
NASA NTRS1971

An improved ion thinning apparatus

Ion bombardment thinning apparatus for preparing electron microscope foils of inorganic nonmetallic samples including ceramics, minerals and rocks

Christie, J. M.↗